Over the next few years, Aki goes on to become one of the world's most well-respected experts on the Ring, as the situation on Earth goes from bad to worse. The Ring grows so thick it drastically reduces the amount of sunlight that reaches Earth. The resulting climate change causes crop failures and famines that kill a billion people.
So when a crewed spaceship is prepared to visit the Ring to find out what humans can do about the situation, Aki is of course one of the four chosen to make the journey. I found the novel at this point readable but not exceptionally compelling. The first third of the novel reads like your standard Big Dumb Object story, the kind of hard SF that I respect but don't consider to be my favorite subset of science fiction.
My interest perked up considerably when, with 2/3 of the book left to go, the Earth people's mission to the Ring turns out to be unexpectedly productive. The Ring is destroyed, and Earth is saved. For the time being, that is. Aki and crew return to an Earth which now needs to prepare itself, logistically and psychologically, for the possible arrival of possibly very angry aliens.
This is where Usurper of the Sun becomes the sort of speculative fiction I like. Glimpses of an Earth simultaneously giddy with accomplishment and very apprehensive about the future struck me as fairly realistically done.
What really intrigued me, though, were the philosophical questions raised by the utter inscrutability of the aliens. This was some of the best speculation I've seen yet about aliens who really might not think like human beings. Rather than aliens who merely don't appear to come from an Anglo-Saxon-derived culture, which is what we get all too often in science fiction.
(Note to future SF writers: The mindset of an actual alien from another star system is likely to be less comprehensible to the average human, than the mindset of a human with autism. Autistic people are human too. Space aliens aren't. Housuke Nojiri gets this.)
I wasn't terribly happy when, at novel's end, the aliens turned from incomprehensible to comprehensible far too easily. I know it wrapped up the novel nicely, and to his credit, the author did justify it pretty well in-universe. It's just... after so much talk devoted to how alien the aliens were, I felt like they shouldn't have been rendered knowable so quickly.
That said, the philosophical musings of the final two thirds were enough for me to enjoy Usurper of the Sun. I'm well aware this recommendation probably won't get people rushing in droves to buy the book. (Unlike certain extraterrestrial civilizations, I'm not mind-blind.) But hey, it's what I enjoyed.